Pure rotational spectra of the IO radical have been observed for vibrational levels up to v =13 in the X-1 (2)Pi (3/2) state and up to v =9 in the X-2 (2)Pi (1/2) state. Isotopically enriched (IO)-O-18 rotational spectra have been observed for vibrational levels up to v = 5 in both the X-1 and X-2 states. These are the first high-resolution spectra of any kind reported for the X-2 state and greatly extend the available data for the X-1 state. The data for both isotopomers have been fitted simultaneously to a single set of (2)Pi parameters with fixed isotopic ratios. The isotope relations among the parameters have provided a means of decorrelating the electron spin-rotation constant gamma from the fine-structure centrifugal distortion constant, A(D), and have allowed the first determination of an effective value for gamma. The rotation-vibration constants correspond to the equilibrium molecular properties r(e)(X-1) =186.762 pm, r(e)(X-2)=188.468 pm, omega (e)(X-1)=681.69 cm(-1), and omega (e)(X-2)=645.29 cm(-1). These constants have been used to calculate X-1 and X-2 Rydberg-Klein-Rees potentials encompassing energies up to 40% of the dissociation limit. A complete set of hyperfine coupling constants has been determined for the first time and interpreted using appropriate relativistic atomic radial integrals.